1. Field of the Invention
This invention relates to a web transfer apparatus for transferring a web from a press section to a dryer section. More particularly, this invention relates to a web transfer apparatus for transferring a paper web from a press section to a paper dryer section so that the unsupported distance travelled by the web between the press and the dryer section is minimized.
2. INFORMATION DISCLOSURE STATEMENT
In order to understand the advantages of the present invention, it is necessary to appreciate the problems involved in guiding and, more particularly, in threading a paper web from a press section to--and through--a dryer section.
The transfer of the wet web from the press section to the dryer section of a conventional paper machine is generally accomplished by first transferring a narrow portion of the web to the dryer section. The next step is that of threading that narrow portion of the web through the entire dryer section. When this has been accomplished, the web width is gradually increased until the entire web runs through the dryer section.
In order to understand the aforementioned operation more particularly, reference is made hereinafter to the aforementioned threading operation using a conventional Tri-Vent press section and BelRun dryer section. The Tri-Vent and BelRun sections are both manufactured by Beloit Corporation.
First of all, the transfer from the press section to the dryer section begins after the full width web has been transferred through the press section and is being removed from the press roll by a doctor blade. This full-width web is first cut longitudinally by a suitable water jet prior to the press section so that a small portion of the front edge of the web is separated from the rest of the web. This small portion of the web is typically 2 to 10 inches (5.08 to 25.4 centimeters) in width and is called the "tail". The tail is then blown off the press roll and directed into a nip defined by the dryer fabric and a first dryer of the dryer section. Currently, there are several devices in common usage for blowing the tail off of the press roll and these devices usually use pressurized air for directing the tail towards the dryer section.
There are two main problems associated with the foregoing operation. The first problem is the difficulty in accurately aiming the nozzle of the pressurized air device so that the tail will get caught in the nip of the dryer and dryer fabric. This problem is aggravated by having long open spans between the press roll and the nip. A long span not only increases the directional accuracy required in blowing the tail off the press roll, but also increases the susceptibility of the tail to being disturbed by local air currents. The second problem results because the tail must be blown off the press roll before it contacts the doctor blade. However, if the tail is blown off too far above the doctor blade, it will not have a direct path to the nip due to the disposition therebetween of the lead-in roll of the dryer fabric. Therefore, the zone or "target" within which the tail must be removed from the press roll is limited in size and such removal, therefore, requires great skill on the part of the operator of the air nozzle to correctly remove the tail within this target and to direct such tail towards and between the nip.
Also, there are several other factors which influence the trajectory of the tail from the press roll which makes this portion of the transfer particularly difficult. To enumerate some of these difficulties, there exists a tendency of the web to fall due to gravity. Furthermore, there is a tendency on the part of the tail, to slow down due to air friction.
In addition to the aforementioned problems regarding the actual threading of the tail between the press section and the dryer section, another major problem exists in that when the tail has been threaded, a certain slack in the tail will exist between the press and dryer sections. Once the tail is caught in the nip, the slack in the tail length must be quickly reduced in order to keep the tail from drooping downwardly and coming into contact with the doctor blade or being broken by local air currents. This slack is reduced by running the dryer section at an increment of speed above that of the press section. This increment is called the "draw". The "draw" must be large enough during this operation to quickly take up any slack in the tail, but if this "draw" is too large, the tail will be broken once the slack is removed. Typically, such "draws" are in the range of 2 to 5 percent of the press speed.
The aforementioned problem relative to the "draw" is also aggravated by having a long span, or distance, between the press and dryer sections. A longer span obviously increases the amount of slack which must be quickly taken up by the dryer section.
In order to overcome the aforementioned problems associated with the prior art proposals, Applicant has reduced the length of this span. Although this solution may, at first, be thought to be relatively simple, in practice--when a portion of the dryer section is located inside the framing of the press section, various additional problems arise. These problems include restrictions in the press frame design, various stresses imposed on the press frame, vibrations and access to the press frame. Also, access to the dryer felt and roll removal becomes very difficult. Because of these difficulties in design, it is not uncommon for the distance between the press section and dryer section to be in the range of 8 to 10 feet. (2.44 meters to 3.05 meters)
In an attempt to stabilize the tail on the dryer fabric during threading, it has been found that if the tail maintains good contact with the fabric, it will not be subject to external air currents and will later progress down the dryer section without wandering.
In order to maintain such contact between the tail and the dryer fabric, several devices have previously been installed above the dryer fabric in order to generate and maintain such good contact. These devices include air nozzles and transfer boxes. Also, diverging foil sections have been used as well as vacuum boxes and air foil devices. These tail stabilizing devices cannot effectively support the narrow tail because the vacuum levels created by such transfer boxes or the like, are not adequate. Such inadequate vacuum is caused by the large open area on each side of the tail and the permeability of the fabric which allows air to flow in above the tail and reduce the effective vacuum at the felt surface. While it is true that the vacuum levels can be increased to support the tail, this increased vacuum will then pull the dryer fabric into contact with the transfer box, or the like when the fabric is later sealed by the full-width web.
The aforementioned long span, or distance travelled by the tail between the press roll and the dryer section, is aggravated because such deflection of the dryer felt due to application of vacuum to the transfer box increases almost by a factor of four with the span length.
With regard to the use of air nozzles for maintaining close contact between the tail and the dryer fabric, these air nozzles have proven to be effective for relatively low speed operations but the spent air is found to cause more disruptions and movement of the tail at higher speeds.
Regarding a further aspect of transferring the web from the press section to the dryer section, once the tail has been transferred to the dryer fabric, it becomes necessary to thread the tail from the first dryer cylinder down the dryer section to the last dryer cylinder. This can be accomplished either with the use of threading air jets or, more commonly, with the use of threading ropes.
When using threading ropes, the tail must be separated from the dryer fabric and either manually, or automatically moved towards the front (or edge) of the machine until such tail is caught between the threading ropes. Although this aspect of the transfer is not usually a problem, the design of the press to dryer transfer apparatus must have provisions for this operation.
More specifically, there must be at least a short, open draw with enough access to move the tail from the dryer fabric so that the tail can be moved into the rope nip.
Once the tail has been threaded down the dryer section, the next step is to widen the tail to the full width of the web. During this widening process, good contact must be maintained between the dryer fabric and the tail. If this is not the case, the tail will become slack and will break.
The tail is particularly susceptible to separating from the dryer fabric at the edges of the tail where the transfer box device is least able to maintain the required support vacuum. And--once again, the long span between the press and dryer sections aggravates this problem by increasing the weight of the unsupported tail.
The final step in transferring from the press to the dryer section includes continuing the support of the full width web. Here again, the edges of the web are the most difficult part of the web to support. With long spans, there exists a limit on the amount of vacuum that can be applied or induced above the fabric because of the aforementioned fabric deflection. Further, the long span also increases the weight of the unsupported web thereby causing it to stretch and increasing the time for the viscoelastic web to relax and lose its internal tension.
As can be appreciated by those skilled in the art, there exist several problems as enumerated herein which present themselves when attempting to transfer a web from a press to a dryer section.
The present invention seeks to overcome the aforementioned inadequacies of the prior art proposals by providing a web transfer apparatus which includes a secondary, or transfer fabric, which cooperates with the dryer fabric such that the web during the majority of the travel between the press to the dryer section is supported simultaneously from both sides.
Therefore, it is a primary objective of the present invention to provide a web transfer apparatus in which the provision of a transfer fabric effectively reduces the length of the unsupported span.
A further objective of the present invention is the provision of a web transfer apparatus in which the guide roll for the transfer fabric is located relatively close to the lead-in roll of the dryer fabric thereby reducing the span to a minimum.
Another object of the present invention is the provision of a web transfer apparatus in which the guide roll bears only slightly against the dryer fabric in order to minimize the angle of wrap of the web as the web moves along the dryer fabric and then between the felts or fabrics in the transfer section.
Another object of the present invention is the provision of a web transfer apparatus in which the transfer apparatus is provided with a second air deflector to insure that the tail follows the dryer fabric as the tail emerges from the transfer section.
These, and other objects of the present invention, will be readily apparent to those skilled in the art by a consideration of the detailed description hereinafter and by reference to the attached drawings.
Although the present invention is described with particular reference to transferring a web between a Tri-Nip press section and a UNIRUN dryer section, it should be appreciated by those skilled in the art that the present invention is not limited by the description of the preferred embodiment. Tri-Nip is a registered trademark of Beloit Corporation. Rather, the present invention is defined by the appended claims which envisage various modifications and applications which fall within the spirit and scope of the present invention as defined by the appended claims.